There is already a virtual currency without a block chain! Safecoin is a part of MaidSafe:

"Safecoin is the currency of the SAFE network and a mechanism to incentivise and reward end users and developers as well as provide access to network services. End users who provide their unused computing resources to the network, called Farmers are rewarded in safecoin, while application developers, called Builders earn safecoin in proportion to how often their applications are used. ...  Safecoins are managed by the network’s Transaction Manager. This is the SAFE equivalent to the block chain, however, in SAFE’s case it is unchained, keeping record of only the existing and previous owner. In this respect, safecoin should be thought of as digital cash." -- http://maidsafe.net/safecoin

Could be a brilliant solution. I haven't learned about it enough yet. Looks at face value to have a huge potential.